Such bellows is produced from thin thermoplastic elastomers using hollow body production techniques. Such bellows has a relatively small diameter collar, a relatively large diameter collar and a bellows device disposed between these collars, with an inner envelope and an outer envelope that decreases in diameter towards the smaller diameter collar.
Such bellows are used above all in passenger vehicles. The material is not rubber but a material of the Hytrel type (registered trademark of Dupont). The walls of the bellows are thin. In their magnitude, they weight 80 to 90 grams. If such a bellows were to be made from rubber, then it would weigh, for instance, 140 g. The material used is expensive. It costs 18 DM per kg. It is true that rubber would be cheaper, but it would not satisfy the demands in the same way. The demands are numerous in many directions: the steering deflection on the front wheel axle amounts to 43 to 46 degrees. The bellows must follow this steering deflection. This is also true when the vehicle is moving. Then, the tendency of bellows is no longer to return to their original position, being to a certain extent pretensioned in the wrong direction. The axles of passenger cars can turn at 1500 r.p.m. The bellows have also to perform the steering turn-in as well as the speeds of revolution.
Also, the bellows provided on the gearbox--farther inwards--nevertheless have to withstand a spring travel of about 15 degrees.
From the temperature point of view, too, the bellows are subject to heavy loadings. They have to remain usable at temperatures from +140 degrees Celsius to -40 degrees Celsius. The -40 degrees C naturally comes from the outside temperature. The +140 degrees C comes from the fact that particularly where the inner bellows are concerned, the heat radiated from the engine is an important factor.
The bellows must also withstand the gravel test, in which gravel is shot at the bellows, in the same way as occurs, for example, when a vehicle is running on crushed chippings. Then, furthermore, the bellows has to withstand chemical loadings caused by grease and oil. Furthermore, the material must not flow when clips are applied to the smaller diameter collar and the larger diameter collar. The bellows must be of such a form that mass production is possible. At present, about five per minute can be produced in multiple molds.
On top of all this, the bellows must withstand considerable flexion work, and also when the bellows has been fitted with alignment errors which can never be entirely avoided. Although the bellows ought not to have any initial tension, when the steering lock is 0 degrees and when the spring travel is in the neutral range, installation errors nevertheless occur which will occasion initial tension anyway and a bellows must be capable of withstanding this too.